Ground and structure motions were recorded on 17 channels at Pacoima Dam
during a magnitude 4.3 earthquake on January 13, 2001. These data are examined by
system identification and finite element modeling to quantify the effects of nonuniform
ground motion and the level of damping exhibited by the dam. The system identification
study indicates the dynamic response of the dam is due mostly to two modes which have
frequencies close to 5hz and damping in the range of 4% to 7% of critical. The
displacement histories of the dam are reproduced fairly well by the finite element model,
but differences in amplitudes for some of the channels remain to be resolved. In addition,
more work is needed to obtain confidence in the results, especially the relatively high
level of damping which seems to be present.